As shown in FIG. 1, a rotor 4 is combined with a stator to construct a rotating machine. The stator has core 1 and a coil 2, with the coil 2 wound around core 1, and is engaged within a frame 3. As shown in FIG. 2, a large-diameter portion 5a is formed substantially at the axial center of a shaft 5 of the rotor 4. A pair of positioning rings 7 are engaged with both axial ends of the large-diameter portion 5a. The positioning rings 7 are formed, for example, of a nonmagnetic material such as aluminum or stainless steel, and a plurality of projections 7a extend axially from the opposed positions of the positioning rings 7. Sets of three permanent magnets 6 are provided on the large-diameter portion 5a of the shaft 5, between the positioning rings 7. Each set of permanent magnets 6 is disposed on the outer peripheral surface of the large-diameter portion 5a of the shaft 5, limited by two pairs of adjacent projections 7a, positioned in the circumferential direction of the shaft 5. Each set of three permanent magnets 6 is formed with an arcuate cross section and is disposed along the axial direction of the shaft 5.
A bonding agent 8 for bonding the permanent magnets 6 to the shaft 5, as shown in FIGS. 1 and 3, fills not only the spaces between the permanent magnets 6 and the large-diameter portion 5a of the shaft 5, but also the space between the permanent magnets 6 and the positioning rings 7.
Recently, there has been an acceleration in developments to reduce the size of various devices and machines. Work to decrease size and increase output power has likewise been proceeding in the field of rotor-equipped rotating machines, giving rise to the problem of high temperatures in these machines. Since a bonding agent fills the space between the permanent magnets 6 and the positioning rings 7 in a rotor of conventional construction, no air gap is provided to allow the permanent magnets 6 and the positioning rings 7 to expand when they heat up due to the temperature rising in the machine. Consequently, excessive stress is produced in the compressing direction of the permanent magnets 6, causing them to crack, with the risk that they may eventually separate and scatter from the shaft 5.
In a cooling process in which the permanent magnets 6 are formed and a motor stops, tensile stress is generated in the permanent magnets 6 due to the contraction of the bonding agent 8 the expansion coefficient of which is great, causing the permanent magnets 6 to crack, with the risk that they may eventually separate and scatter from the shaft 5.
Recently, an increase in the rotor speed of rotating machines has further been required, in addition to the reduction in size and the increase in output power. As the rotational speed of the rotor is increased, the centrifugal force further increases, as does the temperature in the machine. A rotor for use under such conditions must therefore possess sufficient thermal and mechanical strength. However, there is an irregularity in bonding strength, when bonding permanent magnets to a shaft, due to the contamination of the bonding surfaces and the curing conditions of the bonding agent, thereby preventing the attainment of sufficient reliability in the unit's strength. Thus, there might arise the problem of the permanent magnets separating and scattering, due to centrifugal force, during rotation.
It has recently been proposed to rigidly bond permanent magnets to a shaft by winding an insulating film, for example, around the outer peripheral surfaces of the magnets to be bonded, while simultaneously applying a strong tension to the film. However, the film weakens when an external force is concentrated on one point. Thus, pinholes occur at the portions of the film in contact with the corners of the magnets, and the film may crack from these pinholes, over its entirety at the winding time or when the rotor rotates. Therefore, the winding of the film must be done very carefully, thereby resulting in low workability and the problem of reliability of the reinforcement.
Heretofore, the method for manufacturing a rotor formed by bonding permanent magnets has included, in general, coating an ambient temperature-curable or thermosetting bonding agent of epoxy resin on the outer peripheral surface of a shaft, placing permanent magnets of arcuate cross section on the coated surface, and retaining the magnets by a jig until the agent is cured.
However, since the ambient temperature curable bonding agent takes as long as 3 to 16 hours to cure, it is difficult to automate the manufacturing steps, thereby resulting in low productivity, and the requirement of a number of implements for retaining the magnets during the long curing period. Thus, there also arises the problem of high manufacturing cost. The thermosetting bonding agent has a shorter curing time than the ambient temperature-curable bonding agent, but it also takes a considerably long time, and thus has the same problem as above. Further, since the agent requires a curing heating furnace, its facility cost increases, thereby raising the manufacturing cost. A method has been considered for shortening the curing time by using an ambient temperature-curable bonding agent which is enhanced, for example, in reactivity, so as to eliminate the above-mentioned problems, but the availability period of the agent is shortened by the fast reaction speed, and the rapid work of a skillful technician is required. Thus, this method lacks universality for general use.